1. Field of the Invention
The present invention relates generally to a microstrip antenna, and in particular to a spiral inductor embedded in the microstrip antenna that reduces the resonant frequency of the microstrip antenna or shortens the length of the microstrip antenna.
2. The Prior Arts
Nowadays, on the mobile phone market, in addition to the new generation of 3G cellular phones, the GSM and PHS cellular phones are the most popular standards. Due to its high transmission power and superior sound quality, the GSM cellular phone is preferred by the customers over the PHS cellular phone, thus having the largest market share of the mobile phone handsets at the present time. However, the intense electromagnetic radiation emitted by the GSM handsets poses a health problem, so that the PHS cellular phone is gradually getting a sharper competitive edge over GSM cellular phone, thus obtaining a larger market share.
With the rapid development and popularization of the mobile phone, various new types of PHS cellular phones having the features of lightweight, thin profile are developed and put into the market. As such, the cellular phone manufacturers are committed to the development of the reduced-size microstrip antenna for use in the PHS cellular phone. In this respect, various designs have been proposed to reduce the size of the microstrip antenna, thus reducing the size of the handset. However, one of the feasible ways to reduce the size or shorten the length of the microstrip antenna without affecting the resonant frequency is to increase the inductance of microstrip antenna per unit length by increasing phase variation of the current flowing through the inductor.
Usually, the method of increasing the current phase variation is to etch narrow and long slots at the same interval at both sides of a slot antenna, and short-circuit both ends of the slot antenna, thereby increasing the inductance per unit length of the slot antenna and shortening the wavelength of the electric field in the slots. Therefore, the slot antenna may be operable at a lower frequency with the original size. In the other way, a spiral slot inductor may be attached to the end of a resonant slot antenna to lengthen the path of the electromagnetic field, thereby reducing the resonant frequency or shortening the length of the antenna. In another way, cutting the original microstrip antenna into two portions, and then connecting the two portions together by a metallic wire or a lumped inductor can lengthen the path of the current flowing through and generate the phase variations when the current flows through the lumped inductor, thereby lowering the resonant frequency or reducing the size of the microstrip antenna.
Based on the principles mentioned above, the microstrip antenna having an embedded spiral inductor according to the present invention is disclosed.